With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the fabrication process of the multilayer interconnect with finer circuit, as the number of interconnect levels increases, film coverage of step geometry (or step coverage) is lowered in thin film formation because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnect, it is necessary to improve the step coverage and planarize the surface. It is also necessary to planarize semiconductor device surfaces so that irregularity steps formed thereon fall within a depth of focus in optical lithography. This is because finer optical lithography entails shallower depth of focus.
Accordingly, the planarization of the semiconductor device surfaces is becoming more important in the fabrication process of the semiconductor devices. Chemical mechanical polishing (CMP) is the most important technique in the surface planarization. This chemical mechanical polishing is a process of polishing a wafer by placing the wafer in sliding contact with a polishing surface of a polishing pad while supplying a polishing liquid containing abrasive grains, such as silica (SiO2), onto the polishing surface.
A polishing apparatus for performing CMP has a polishing table that supports the polishing pad having the polishing surface, and a substrate holder for holding the wafer. The substrate holder is also called a top ring or a polishing head. This polishing apparatus polishes the wafer as follows. The top ring holds the wafer and presses it against the polishing surface of the polishing pad at predetermined pressure. The polishing table and the top ring are moved relative to each other to rub the wafer against the polishing surface to thereby polish a surface of the wafer.
When polishing the wafer, if a relative pressing force applied between the wafer and the polishing pad is not uniform over the entirety of the surface of the wafer, insufficient polishing or excessive polishing would occur depending on the pressing force applied to each portion of the wafer. Thus, in order to even the pressing force exerted on the wafer, the top ring has at its lower part a pressure chamber formed by a flexible membrane (or a membrane). This pressure chamber is supplied with gas, such as air, to press the wafer against the polishing surface of the polishing pad through the membrane under the gas pressure.
After polishing of the wafer is terminated, the wafer on the polishing surface is attracted to the top ring via vacuum suction. The top ring is elevated together with the wafer, and is then moved to a position above a transfer stage. The top ring then releases the wafer from the membrane. The release of the wafer is achieved by ejecting a release shower into a gap between the wafer and the membrane while supplying the gas into the pressure chamber.
When the wafer is released from the membrane, the membrane may expand largely due to deterioration of the membrane. If the membrane expands largely, the release shower does not reach a contact portion of the wafer and the membrane. As a result, the wafer may not be released from the membrane.